1. Field of the Invention
This application relates generally to two-way ranging and more specifically, but not exclusively, to a messaging scheme for two-way ranging.
2. Relevant Background
Distance ranging involves determining a distance between two locations. In a typical scenario, a ranging device measures a distance from the ranging device to another object. Here, the ranging device may determine the amount of time it takes for a signal to travel between the ranging device and the other object. The ranging device may then estimate the distance based on the signal propagation time and the known propagation speed of the signal (e.g., estimated as the speed of light). A ranging device may employ a variety of technologies such as laser, radar, sonar, and various forms of radio-frequency (RF) signaling. For convenience, the term distance ranging will be referred to herein simply as ranging.
In some cases, a two-way ranging scheme may be employed to determine the distance between two devices. FIG. 1 illustrates a simplified example of ranging signal timing for two devices (e.g., wireless devices) performing a two-way ranging operation. Here, a device A may determine the distance to a device B based on a round-trip time associated with signals transmitted by the devices. For example, distance may be estimated based on the equation: D=TP*C, where D is the estimated distance, TP is the signal propagation delay from one device to the other, and C is the speed of light. The signal propagation delay TP may be estimated based on the round-trip time as discussed below.
The signals of FIG. 1 are depicted in a simplified form for purposes of illustration. Here, a device B generates a signal 102 that is transmitted over-the-air (as represented by an arrow 104) to a device A. This signal is received at the device A (as represented by a signal 106) after a propagation time represented by a time period arrow 108. Following a turnaround time period (as represented by a time period arrow 110) after receiving the signal 106, the device A generates a signal 112 that is transmitted over-the-air (as represented by an arrow 114) to the device B. This signal is received at the device B (as represented by a signal 116) after a propagation time represented by a time period arrow 118. Each device generates a timing indication (hereafter referred to, for convenience, as a timestamp) associated with the transmission and reception of these signals. For example, the devices A and B may record transmit timestamps at T3A and T1B, respectively, and the devices A and B may record receive timestamps at T2A and T4B, respectively. Based on these timestamps, an estimated propagation delay TP (e.g., corresponding to time period arrow 108 or 118) may be computed. For example, a round-trip time estimate may be determined according to: 2TP=(T4B−T1B)−(T3A−T2A). Here, T1B, T2A, T3A, and T4B are measurable. In addition, device B may send to device A an indication of the time period between T1B and T4B (represented by a time period arrow 120) measured by device B. Consequently, device A may calculate the round-trip time based on the indication received from device B (the time period arrow 120) and the turnaround time measured by device A (the time period arrow 110).
In the two-way ranging scheme described in FIG. 1, various types of information are sent from one device to another. Accordingly, a need exists for an effective technique for exchanging this information and/or other similar information to accomplish two-way ranging.